Pigment bleeding, in the context of printing inks, is an undesirable color shift that occurs in printed ink due to sensitivity to over-print coatings or post-processing treatments. It's essentially when the pigment in the ink migrates or dissolves, leading to a change in the original color or appearance.
Here's a more detailed breakdown:
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The Problem: Standard pigments used in commercial inks aren't always robust enough to withstand the application of various coatings (like varnishes or laminates) or other post-printing processes (like heat sealing or chemical treatments).
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What Happens: When these sensitive pigments come into contact with such coatings or treatments, they can react in a few ways:
- Dissolving: The pigment particles might dissolve partially or completely into the coating.
- Migration: The pigment particles might migrate out of the ink film and into the coating or substrate.
- Chemical Reaction: The pigment might undergo a chemical change, altering its color.
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The Result: This reaction causes a noticeable color change, which is the "bleed" or "burn-out" effect. This can manifest as:
- A fading of the original ink color.
- A change in hue (e.g., a red ink becoming more pink or orange).
- A halo or discoloration around the printed area.
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Examples:
- A red ink bleeding into a clear varnish, causing the varnish to turn pink.
- A blue ink fading or shifting towards green after being subjected to heat sealing.
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Prevention: The best approach is prevention, involving:
- Pigment Selection: Choosing pigments that are known to be resistant to the specific coatings and processes being used. Consider using more robust, higher-quality pigments.
- Ink Formulation: Formulating inks with appropriate binders and additives that minimize pigment migration and dissolution.
- Coating Selection: Selecting coatings that are compatible with the inks being used.
- Testing: Thoroughly testing ink/coating combinations before production to identify potential bleeding problems.
- Protective Overprints: Use of a clear protective layer between the printed ink and the final coating can sometimes prevent issues.
In short, pigment bleeding is an undesirable color shift in printed inks caused by the pigment's sensitivity to over-print coatings or post-processing treatments, resulting in color changes and compromised print quality. Selecting appropriate pigments and coatings, along with thorough testing, is key to avoiding this issue.
